Zeolites are extensively used as adsorbents, catalysts for acid reactions, oxidation reactions, etc., separation materials, optical materials such as quantum dots and quantum wires, fine semiconductor particles serving as a magnetic material, and host materials for a fluorescent substance, dye, etc. The zeolites include crystalline silicates, crystalline aluminophosphates, and the like according to International Zeolite Association (hereinafter referred to as IZA).
A zeolite is usually produced by using raw materials for the zeolite and a structure-directing agent (referred to also as organic template) to conduct hydrothermal synthesis and then removing the organic template by a technique such as burning or extraction. In general, zeolites from which the organic template has been removed as much as possible are put as industrially useful zeolites to use as catalysts, adsorbents, etc. The reason why the organic template is removed as much as possible is as follows. In the zeolite which has been obtained by, e.g., hydrothermal synthesis and from which the organic template has not been removed (hereinafter referred to as zeolite precursor), the spaces in the pores are occupied by the organic template to prevent a substrate for reaction or a substance to be adsorbed from coming into the pores. This zeolite cannot hence function as a catalyst or adsorbent. Consequently, the zeolites which are industrially useful are usually ones obtained by removing the organic template from zeolite precursors as much as possible.
A process for producing, in particular, a silicoaluminophosphate containing silicon as a heteroatom, among zeolites, is described in patent document 1 shown below. This process is a method in which when an organic template is removed from a zeolite precursor for a silicoaluminophosphate, the zeolite precursor is heated in a stream of air to decompose and remove the organic template (burning method). The document includes a statement to the effect that SAPO-5 (framework density (FD): 17.3 T/nm3) having a carbon content of 6.9% by weight before burning was produced. However, the SAPO-5 obtained here has a relatively high framework density and our investigations revealed that it has an insufficient adsorption when used as an adsorbent.
Patent document 2 shown below teaches that in the case of a zeolite whose pore structure is partly occupied by either an organic template or a product of organic-template decomposition, the catalytic activity thereof does not decrease during storage or transportation because the adsorption of atmospheric moisture is prevented. Examples given therein include one in which carbon atoms remain in an amount of 8.20% by weight or larger. On the other hand, the document teaches that when the zeolite is to be used as a catalyst, it is subjected to a burning or combustion treatment in an oxygen-containing atmosphere as an activation treatment to completely remove the organic template or the like from the pore structure of the zeolite.
A process for producing, in particular, an iron aluminophosphate containing iron as a heteroatom, among zeolites, is described in patent document 3 shown below. This process is a method in which when an organic template is removed from a zeolite precursor for an iron aluminophosphate, the zeolite precursor is heated in a stream of air or stream of nitrogen to decompose and remove the organic template (burning method).
On the other hand, the present applicant previously proposed zeolites suitable for use in a heat utilization system which have a framework density (FD) of 10-18 T/nm3 and give an adsorption isotherm showing specific adsorptive properties. More specifically, an iron aluminophosphate, a silicoaluminophosphate, and an iron silicoaluminophosphate each having those properties were proposed (patent document 4 shown below).
Furthermore, in non-patent document 1 shown below, there is a report that a silicoaluminophosphate (SAPO-43) having a GIS structure containing carbon atoms and nitrogen atoms remaining in amounts of from 3.99% by weight to 6.09% by weight and from 4.83% by weight to 6.09% by weight, respectively, shows the ability to selectively adsorb CO2, H2O, H2S, etc. However, it is pointed out therein that this zeolite from which the organic template has been completely removed is intrinsically unstable and that destruction of the crystal structure proceeds with the removal of the organic template and the complete removal of the organic template almost completely deprives the zeolite of its adsorbing ability. When this zeolite was prepared so that the organic template remained to enable the zeolite to retain its structure, the adsorption capacity of the zeolite obtained was as low as 8% by weight in the case of water adsorption.
[Patent Document 1]
U.S. Pat. No. 4,440,871
[Patent Document 2]
U.S. Pat. No. 6,395,674
[Patent Document 3]
U.S. Pat. No. 4,554,143
[Patent Document 4]
EP 1391238
[Non-Patent Document 1]
R. T. Yang et al., Langmuir, 2003, 19, 2193-2200